Common for all organic functional devices, such as organic light emitting diodes (OLEDs), organic solar cells, organic photovoltaic elements, organic photo diodes, organic photosensors etc., is that an organic functional layer is sandwiched between and interacts with a pair of electrode layers. In an OLED, the application of a voltage between the electrode layers results in emission of light by the organic functional layer and in an organic solar cell, absorption of light by the organic functional layer leads to the creation of a voltage between the electrode layers.
For organic functional devices that interact with light, including organic LEDs, organic solar cells etc, at least one of the electrode layers is generally transparent. Transparent electrode layers typically have a relatively large surface resistance (for an ITO (indium tin oxide) layer, the surface resistance is typically about 10-15Ω/). Due to this high surface resistance, the voltage between the electrode layers sandwiching the organic functional layer becomes dependent on lateral position across the device. This position dependent voltage leads to a non-uniform distribution of the current flowing through the organic functional layer in the device. For an OLED, this leads to a non-uniform light output of the OLED.
In order to improve the uniformity of the current distribution in an organic functional device, highly conductive shunting structures are often provided to distribute an electrode layer voltage over an electrode layer having a high surface resistance.
However, materials having a sufficiently high conductivity for use in shunting structures are opaque and can therefore not be deposited on too large coherent portions of the electrode layer.
A standard method to solve that is to make use of a metal grid. For transparent electrodes fabricated before the organic functional layer is provided, this is normally rather straight-forward, since various dry and wet deposition techniques, such as photolithography, as well as a high deposition temperatures can be used, whereby complex patterns can be formed, and a wide variety of materials are available for deposition.
Following provision of the organic functional layer, however, the number of suitable deposition techniques is severely limited, since the organic functional layer is sensitive to high temperatures etc. A possible method for applying shunting structures without damaging the organic functional layer is to use a shadow mask and evaporate a highly conductive substance, such as a metal or carbon through openings in the mask. Due to the inherent design constraints of a shadow mask, the shunting structures deposited through this method are, however, limited to isolated, rather simple structures, such as substantially parallel lines.
Through the provision of such structures, the uniformity of the current distribution in the organic functional device is improved, especially in the general direction of extension of the shunting structures. In portions of the organic functional device between such isolated structures there are typically, however, still variations in the voltage across the organic functional layer, and accompanying non-uniformity in the current distribution.